1. Field of the Invention
The present invention relates to a method of bleaching a thermosensitive recording medium by exposing it to ultraviolet rays, and more particularly to a method of bleaching a clamped portion of the thermosensitive recording medium which is clamped at its one end by a clamp member so as to be secured to the periphery of a platen drum.
2. Related Art
There are generally two types of thermal printers: one type is for direct thermal recording using thermosensitive recording media and the other type is for thermal transfer recording including wax thermal transfer recording and thermal dye transfer recording. Since the direct thermal printer produces no waste such as a dye transfer sheet and requires a lower running cost, the direct thermal printers tend to be used more frequently. Recently, a thermosensitive color recording medium has been suggested, for example, in JPA 61-213169, which has three thermosensitive coloring layers for developing cyan, magenta and yellow colors formed one after another, so that full-color images can be recorded thereon by using direct thermal printers.
In the thermosensitive color recording medium, thermal recording is sequentially performed from the upper or outermost coloring layer to the lower or innermost coloring layer. After recording each of the coloring layers, electromagnetic rays having a wavelength range which is specific to each coloring layer are projected onto the recording medium, for optically fixing the just-recorded coloring layer, so that the coloring layer is not repeatedly recorded during the following thermal recording process.
Therefore, in a conventional direct thermal printer, a thermal head and two ultraviolet lamps for optically fixing the yellow and magenta coloring layers are disposed in opposition to the periphery of a rotational platen drum, and the thermosensitive color recording medium, such as a sheet of thermosensitive color paper is wound around the platen drum and secured thereto by a clamp member. The clamp member clamps a portion of the thermosensitive color paper, mostly, the leading end thereof, so as to prevent slippage of the color paper during the printing.
Because the thermosensitive color paper originally is pale yellow and the portion of the color paper clamped by the clamp member is not recorded nor fixed during the printing, it is necessary to expose the clamped portion to ultraviolet rays after the printing or during paper ejection, so as to bleach the clamped portion. Thereby, the clamped portion is also optically fixed and prevented from being undesirably colored by extraneous heat energy. If the thermosensitive color paper has a transparent base material, non-recorded portions of the coloring layers are made colorless and transparent by the bleaching.
However, to bleach the clamped portion sufficiently, the thermosensitive color paper should conventionally be transported under the ultraviolet light at a very low speed. That elongates the total time necessary for accomplishing the printing.